A nanogenerator is any nanoscale device used to generate power. Usually this would mean dimensions less than a micron (1000 nanometers or 1/1000th of a millimeter), but in practice, the word has been used to describe generators as large as 2 mm, or 2,000,000 nanometers in diameter. Alternatively, a nanogenerator may be any generator that uses nanoscale tools, like nanowires, to generate electricity.
So far, the term "nanogenerator" has mainly been used in reference to the work of Zhong Lin Wang and colleagues at the Georgia Institute of Technology. Wang's team built a nanogenerator that uses an array of zinc-oxide nanowires to convert ultrasonic waves into electricity. The nanogenerator exploits the piezoelectric effect, a phenomenon displayed by certain materials whereby mechanical stress is converted directly into electric current. Wang visualizes the nanogenerator built into future nanomachines or implants that can get power directly from vibrations within the human body caused by the circulatory system. Dr. Wang's prototype nanogenerator was 2 mm in size, but he hopes to shrink it to nanoscale size and make it ready for adoption by 2011.
When nanoscale machines were first envisioned by Eric Drexler in the late 1970s, one of the main objections that came up regarding their feasibility was "How are these to be powered?" The work of Dr. Wang answered this question when it was presented in 2007. Still, the approach of using piezoelectricity to power nanodevices is still just one among many — other possible approaches to nanoscale power generation or distribution include chemical energy, using nanowires to channel energy from a macroscale source, getting energy directly from heat, or a generator based on electromagnetic waves like UV light.
Though overshadowed by Dr. Wang's team, there has been some other work in nanogenerators. Dozens if not hundreds of academic and industry groups are doing research into nanoscale power generation. Nanoscale power generation may offer orders of magnitude improvements in power generation and storage density relative to current technologies. This could enable electric cars that can run for hundreds or even thousands of miles without a recharge, if the technology advances far enough.